This application claims the benefit of Korean Patent Application No. 1999-56020, filed on Dec. 9, 1999, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a liquid crystal display device implementing in-plane switching (IPS) where an electric field to be applied to liquid crystal is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
Recently, liquid crystal display (LCD) devices with light, thin, low power consumption characteristics have been used in office automation (OA) equipments and video units and the like. Typically there are two types of LCDsxe2x80x94a twist nematic (TN) mode and a super twist nematic (STN) mode. Although TN-LCDs and STN-LCDs have been in wide use, they have a drawback of a very narrow viewing angle. In order to solve the problem, IPS-LCD devices have been proposed. An IPS-LCD device includes a lower substrate where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode and a liquid crystal is interposed between the upper and lower substrates.
As shown in FIG. 1, lower and upper substrates 1a and 1b are spaced apart from each other, and a liquid crystal xe2x80x9cLCxe2x80x9d is interposed therebetween. The lower and upper substrates are called array and color filter substrates, respectively. On the lower substrate 1a, pixel and common electrodes 15 and 14 are disposed. The pixel and common electrodes 15 and 14 are parallel with and spaced apart from each other. On a surface of the upper substrate 1b, a color filter 25 is disposed facing the lower substrate 1a. The pixel and common electrodes 15 and 14 apply an electric field xe2x80x9cExe2x80x9d to the liquid crystal xe2x80x9cLCxe2x80x9d. The liquid crystal xe2x80x9cLCxe2x80x9d has a negative dielectric anisotropy, and thus it is aligned parallel with the electric field xe2x80x9cExe2x80x9d.
FIGS. 2 to 5 conceptually illustrate operation modes of a typical IPS-LCD device. When the electric field is not generated between the pixel and the common electrodes 15 and 14, the long axes of the LC molecules xe2x80x9cLCxe2x80x9d maintain an angle relative to a perpendicular line to the parallel pixel and common electrodes 15 and 14. For example, the angle is 45 degrees.
When the electric field is generated between the pixel and common electrodes 15 and 14, because both of the pixel and common electrodes 15 and 14 are formed on the lower substrate 1a, the in-plane electric field xe2x80x9cExe2x80x9d, which is parallel to the surface of the lower substrate 1a, is generated between the pixel and common electrodes 15 and 14. Accordingly, the LC molecules xe2x80x9cLCxe2x80x9d move to coincide the long axes thereof with the electric field direction, and the LC molecules xe2x80x9cLCxe2x80x9d become aligned such that the long axes thereof is parallel with the perpendicular line to the pixel and common electrodes 15 and 14.
By the above-mentioned operation modes and with additional elements such as polarizers and alignment layers, the IPS-LCD device displays images. The IPS-LCD device has wide viewing angles, low color dispersion qualities, and the fabricating processes thereof are simpler as compared to other LCD devices. But, since the pixel and common electrodes are disposed on the same plane of the lower substrate, the transmittance and aperture ratio are low.
For the sake of discussing the above-mentioned problem of the IPS-LCD device in detail, the structure of the IPS-LCD device will be described in detail with reference to FIGS. 6A and 6B.
FIG. 6A is a plan view illustrating in detail the structure of one pixel region in the IPS-LCD device, specifically, a unit pixel region 10. In addition, a cross-sectional view taken along a line xe2x80x9cBxe2x80x94Bxe2x80x9d in FIG. 6A is illustrated in FIG. 6B.
On the surface of the transparent substrate 1A adjacent to the liquid crystal layer, a gate line (or scan signal line) 2 made of, for example, aluminum (Al) is formed extending along the x-direction. In addition, a common line (or reference signal line) 4 is formed extending along the x-direction, close to the gate line 2 on the +y-direction side thereof. The common line 4 is also made of, for example, Al. A region surrounded by the gate line 2, the common lines 4, and the data lines 3 constitutes a pixel region, as previously described.
In addition, the pixel region 10 includes a common electrode 14 formed by the common line 4, and another common electrode 14 formed adjacent to the gate line 2. The pair of horizontally extending common electrodes 14 are positioned adjacent to one of a pair of data lines 3 (on the right side of FIG. 6A), and are electrically connected to each other through a conductive layer 14A which is formed simultaneously with the common electrodes 14.
In the structure described above, the pair of common electrodes 14 extend in the direction parallel with the gate line 2. In other words, the common electrodes 14 take the form of a strip extending in a direction perpendicular to the data lines 3.
On the surface of the lower substrate 1a on which the gate lines and other lines discussed above are formed, a first insulating film 11 (see FIG. 6B) made of, for example, silicon nitride is formed overlying the gate line 2, the common lines 4, and the common electrodes 14. This first insulating film 11 functions as an inter-layer insulating film for insulating the gate line 2 and the common lines 4 from the data lines 3 as a gate-insulating layer for a region in which a thin film transistor xe2x80x9cTFTxe2x80x9d including a drain electrode 3a and a source electrode 15a is formed. The first insulating film 11 also acts as a dielectric film for a region in which a capacitor Cstg is formed. A semiconductor layer 12 for the TFT is formed near a cross point of the gate and data lines 2 and 3. On the other surface of the lower substrate 1a, a first polarization layer 18 is formed.
On the first insulating film 11, a pixel electrode 15 is formed parallel with the common electrode 14. An end portion thereof is electrically connected with the conductive layer 14a, and the other portion thereof is electrically connected with the source electrode 15a. A first planar film 16 is formed on the first insulating film 11 to cover the pixel electrode 15, and on the first planar film 16, a first alignment film 17 is formed.
FIG. 6B illustrates a cross-sectional view of the upper substrate 1b on which a black matrix 300 is formed. In the opening of the black matrix 300, a color filter 25 is formed to fill the opening. Then, a second planar film 27 is formed to cover the color film 25 and the black matrix 300, and a second alignment layer 28 is formed on the surface of the second planar film 27 facing the liquid crystal layer.
The color filter 25 is formed to define three sub-pixel regions adjacent to and extending along the data line 3 and including a red (R) filter, a green (G) filter, and a blue (B) filter, for example, from the top of the three sub-pixel regions. The three sub-pixel regions constitute one pixel region for a color display.
A second polarization layer 29 is arranged on the surface of the upper substrate 1b opposite to the surface adjacent to the liquid crystal layer on which various films are formed as described above.
In FIG. 6B, a voltage applied between the common electrodes 14 and the pixel electrode 15 causes an electric field E to be generated in the liquid crystal layer LC in parallel with the respective surfaces of the lower and upper substrates 1a, 1b. This is the reason why the illustrated structure is referred to as in-plane switching.
As shown in FIG. 7, if a distance xe2x80x9cLxe2x80x9d between the common and pixel electrodes 14 and 15 becomes longer, the aperture ratio problem can be solved. However, a larger xe2x80x9cLxe2x80x9d causes the threshold voltage to drive the liquid crystal to be higher. That is to say, the threshold voltage xe2x80x9cVthxe2x80x9d is proportional to xe2x80x9cL/dxe2x80x9d, where xe2x80x9cdxe2x80x9d is the width of the pixel electrode 15 (Vth xe2x88x9dL/d). If the distance xe2x80x9cLxe2x80x9d becomes longer, the electric field generated between the pixel and common electrodes becomes weaker. Accordingly, the voltage difference between the pixel and common electrodes needs to be larger for a normal operation of the IPS-LCD device.
However, driving circuits that provide the voltage difference to the electrodes have limitations making it difficult to increase the voltage difference. Accordingly, the distance xe2x80x9cLxe2x80x9d can not be increased to solve the aperture ratio problem.
Accordingly, the present invention is directed to an IPS-LCD device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide an IPS-LCD device having a high aperture ratio.
Another advantage of the present invention is to provide an IPS-LCD device having a low threshold voltage.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a liquid crystal display device including a substrate; a thin film transistor on the substrate; a pixel electrode having a plurality of first tips, the first tips being formed at at least one edge of the pixel electrode; and a common electrode having a plurality of second tips, the second tips being formed at at least one edge of the pixel electrode, and the common electrode being substantially parallel with the pixel electrodes.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.